For the 30th anniversary of the Loma Prieta earthquake—the 6.9 tremor struck October 17, 1989 shortly before game three of the World Series between the Oakland Athletics and the San Francisco Giants—Curbed SF will feature quake-related coverage looking back at that fateful day, and what you can do to prepare for the next big one.
No place in the Bay Area is safe when it comes to the inevitable earthquakes that loom on the horizon. But some neighborhoods are better situated than others.
The California Geological Survey (CGS) updated its seismic hazard zone map in April, revealing the most dangerous places in the Bay Area during phenomena like liquefaction and landslides following a major earthquake.
CGS defines a major earthquake as measuring 5.5 or greater on the Richter scale. CGS explains that hazard zones are areas of particular danger even relative to usual earthquake risks: “Although strong ground shaking is responsible for most earthquake-related damage, these zones identify areas where earthquake hazards other than structural shaking—specifically ground failures during an earthquake—are more likely.”
In liquefaction zones, saturated sand and silt take on the characteristics of a liquid during the intense shaking of an earthquake, according to the U.S. Geological Survey.
During violent quakes, seemingly solid ground can turn into the consistency of cake batter, collapsing overhead buildings and infrastructure.
“The highest hazard areas are concentrated in regions of man-made landfill, especially fill that was placed many decades ago in areas that were once submerged bay floor,” notes USGS.
This means that, according to the map, places like the Marina, the Financial District, the majority of SoMa, South Beach, the East Cut, Treasure Island, Ocean Beach, India Basin, Hunters Point, and Candlestick Point are in danger from the phenomenon.
So too are smaller slices of neighborhoods where liquefaction concerns don’t usually brook as much attention, such as significant portions of the Mission, the Haight, Miraloma, Ingleside Terraces and SF State University, and the Castro.
Outside of San Francisco, the entire city of Alameda lies within a liquefaction zone, as does most of San Leandro and Hayward.
West Oakland and most of Emeryville also face liquefaction risks, as does the western flank of Berkeley.
In the South Bay and along the Peninsula, one massive liquefaction zone extends from SFO down through most of Santa Clara and San Jose.
Map users may search for a physical address to see what kind of quake hazards that property may face. The maps and accompanying app can also be used to determine which potential quake hazards must be disclosed when attempting to sell property in California.
CGS notes that due to limitations in data availability and mapping at regional scales, “it is expected that some of these hazards will produce damaging ground failures outside the delineated zones in future earthquakes.”
But the agency adds that “the majority of occurrences will be within the zoned areas.”
Earthquakes of 5.5 or greater are relatively common (in geologic terms) along Bay Area faults, but may become more or less frequent depending on other recent eruptions.
As CGS researchers wrote in the Bulletin of the Seismological Society of America in 2002:
Cycles of seismicity and quiescence were associated with the Bay area earthquakes of 1868, 1906, and 1989. The 1868 earthquake on the Hayward fault was preceded by 12 earthquakes of M ≥5.5 from 1855 to 1866, within 60 km of the Hayward fault, and was followed by 13 quiet years.
The 1906 San Andreas fault event was preceded from 1881 to 1903 by 18 earthquakes of M ≥5.5 and was followed by quiescence, with only three earthquakes of M ≥5.5 until 1954. The Bay area has been seismically quiet at the M ≥5.5 level since the 1989 Loma Prieta earthquake and its 1990 aftershocks, which contrasts with the 10 years before 1989, when five M 5.5–6.2 events occurred.
The Loma Prieta earthquake is of similar magnitude to the 1868 Hayward event and could be followed by a similarly short quiet period.
Residents can access the info via an online app that compiles the disparate maps together.